Examples of a method of improving the SN ratio (signal to noise ratio) of wireless communication include beam forming in which the energy of a radio signal is concentrated in a direction of a beam by using an array antenna. Among various methods of performing beam forming by using array antennas, a method of inputting digital information multiplied with a weight to a DAC attached to each antenna element allows each antenna element to be freely operated and provides favorable performance. DAC is abbreviation for digital to analog convertor. However, a DAC consumes a large amount of power, and thus use of the same number of DACs as the number of antenna elements increases power consumption.
Hybrid beam forming is known as a method of performing beam forming with a reduced number of DACs. When the hybrid beam forming is applied, an array antenna including a plurality of subarrays is used, and one DAC is used for each subarray. Each antenna element of the array antenna is provided with a phase controller (phase shifter), and the direction of a beam is controlled by controlling the phase of a radio signal through phase control by a baseband unit and phase control by the phase controller.
As examples of the related art, Japanese Laid-open Patent Publication Nos. 60-264104, 1-129508,and 8-181540 and Japanese National Publication of International Patent Application No. 2007-511124.